PROJECT SUMMARY The mosquito-borne flaviviruses have (re-)emerged causing widespread disease with dengue virus and Zika virus as important members. In recent Zika virus outbreaks, it became clear that the virus can cause severe congenital abnormalities in unborn children, posing an immediate threat to public health. Like other positive- stranded RNA viruses, flaviviruses extensively rearrange endoplasmic reticulum (ER) membranes to create a favorable niche for RNA replication. The ER-membrane serves as physical support for the coordinated accumulation of viral and cellular components required for efficient replication. However, little is known about the molecular mechanisms by which viral proteins assemble functional RNA replication complexes and, more specifically, the role of host proteins in this process. Through genome-scale genetic screens, we have discovered that ER-membrane multiprotein complexes, including the oligosaccharyltransferase complex (OST), have critical roles in flavivirus infection. We pinpointed the role of the OST complex to viral RNA replication but unexpectedly found that the enzymatic activity of OST in N-linked glycosylation is dispensable. We demonstrated that the OST complex binds to several nonstructural proteins and is present in complexes associated with viral RNA. Our preliminary data suggest that flaviviruses have evolved to coordinate the assembly of functional RNA replication complexes at the ER membrane through specific interactions between the host proteins and viral proteins/RNA. Two integrated specific aims are proposed to understand the mechanisms by which flaviviruses exploit host factors to establish productive infection. In aim 1, we will use biochemical and genetic approaches to precisely map the protein-protein interactions between individual viral proteins and the cellular factors with a focus on the OST complex. Given the strong dependence on the OST complex, these interaction interfaces may serve as a basis for pharmacological disruption. In aim 2, we will take an RNA-centric approach to systematically uncover cellular proteins that form viral RNA-protein complexes in dengue and Zika infected cells. We will use innovative methods including ChIRP-MS and CLIP- seq, that will provide a detailed and comprehensive overview of the proteins, and even the individual protein domains that bind to the viral RNA. These studies will fundamentally advance our understanding of the molecular mechanisms by which different flaviviruses have subverted ER functions to promote their infectious cycles, and may help to develop therapeutic targets for host-based antivirals.